Micromachining allows the manufacturing of structures and machines so small that they are imperceptible with the human eye. Micromachined devices are commonly used as pumps, motors, accelerometers, pressure sensors, chemical sensors, valves, micro-motion systems, and grippers, and commonly have dimensions on the scale of nanometers to centimeters. Micromachined systems are know in the art as MicroElectroMechanical Systems, or MEMS. MEMS is a relatively new technology that exploits the existing microelectronics infrastructure to create complex machines with micron feature sizes.
An ideal material from which to make MEMS is polycrystalline silicon (polysilicon). Its mechanical properties are suitably strong, flexible, and does not readily fatigue. Additionally, polysilicon is directly compatible with modem integrated circuit fabrication processes. Often, MEMS are produced in batch fabrication, leading to large volumes and extremely low fabrication costs.
Micromachines can have no moving parts, bending parts, or completely free and movable parts. These types of devices have been formed by surface micromachining, bulk micromachining, and LIGA (meaning Lithographie, Galvanoformung, Abformung) (and variations thereof). Surface micromachining is accomplished by three basic techniques: deposition of thin films; wet chemical etching; and dry etching techniques. The most common form of dry etching for micromachining application is reactive ion etching (RIE). Ions are accelerated towards the material to be etched, and the etching reaction is enhanced in the direction of travel of the ion. RIE is an anisotropic etching technique. Trenches and pits many microns deep of arbitrary shape and with vertical sidewalls can be etched by prior art techniques in a variety of materials, including silicon, oxide, and nitride. RIE is not limited by the crystal planes of polysilicon.
Dry etching techniques can be combined with wet etching to form various micro devices. “V” shaped grooves or pits with tapered sidewalls can be formed in silicon by anisotropic etching with KOH etchant. Another etching technique, with roots in semiconductor processing, utilizes plasma etching.
Weak magnetic fields have been used to provide for asymmetric microtrenching using high-density fluorocarbon plasma etching techniques. As practiced in the art, a small magnet can be used at the center of a semiconductor wafer. The wafer would be previously patterned for the subsequent etching procedure. A 1600 nm thick oxide (BPSG) layer over a silicon wafer is patterned with a 900 nm thick resist mask. Prepared in this way, the wafer is etched for 150 seconds at a self-bias voltage of −125 volts (150 W RF bias power) at 3.4 MHz, to a depth of around 100 nm into the oxide layer. The thickness of the mask after etching is about 700 nm. By etching while the plasma is subjected to a magnetic field (−102G), where the magnetic field runs parallel to the wafer cross section, deeper etching is accomplished on one side of a trench than the other as directed by the magnetic field.
It would be advantageous if micromachine devices could be fabricated with increased delicacy and precision. More precise control of etching techniques to create increasingly complex shapes and forms for micromachine devices would be an advantage in the art. Greater control of etching techniques would lead to new types of devices, not available with less precise techniques.